1. Field of the Invention
This invention relates to a control board and more particularly to a high impedance, electrically decoupled set of trace conductors formed along layers within the printed circuit board.
2. Background of the Relevant Art
A backplane or backpanel, referred to herein as a "control board", is a special type of printed circuit board. The control board is sometimes referred to as a mother board in that it provides interconnection with two or more daughter boards coupled to the control board. The control board and daughter boards are all coupled together within an enclosure of an electronic product or system. Like all printed circuit boards, the control board typically comprises one or more layers of rigid laminate material on which printed trace conductors are formed.
Each daughter board may be used to provide mechanical and electrical support for a peripheral controller arranged thereon. The daughter board therefore includes numerous integrated and discrete circuits arranged upon one or both outer surfaces of the daughter board. The are many types of well known peripheral controllers such as, for example, floppy disk controllers, hard disk controllers, tape drive controllers, printer controllers, etc. Each peripheral controller and associated circuitry can therefore be arranged on each daughter board. Additionally, the circuitry of each daughter board is interconnected to other daughter board circuitry via connector stubs arranged in uniform rows across the control board.
Small computer system interface (SCSI) is designed to provide a standard interface between the host system and up to eight peripheral controllers. A peripheral controller associated with one daughter board may be purposefully designed according to the SCSI standard to send and receive signals to and from another peripheral controller arranged on a separate daughter board. The SCSI transmission protocol therefore advantageously allows direct data transferral from one peripheral device over the SCSI bus to another peripheral device without intervention from the host adapter. The host adapter might initiate the transfer, but the transfer of data proceeds directly between the two peripheral controllers. Direct data transferral between peripheral controllers thereby entails trace conductors placed within the control board between the connector stubs. Each connector stub is designed to frictionally engage and electrically connect with edge connectors arranged at one end of a respective daughter board. The connector stubs and interposed trace conductors associated with the control board ensures a SCSI bus direct data transferral.
The trace conductors which provide SCSI bus direct data transferral must be properly configured according to SCSI standards. SCSI standards dictate that the source and load impedances be somewhat matched. This requires that each trace conductors arranged between connector stubs have a closely controlled high impedance value defined in SCSI specification. In addition to impedance requirements, SCSI specification has further defined a minimum spacing between connector stubs. The connector stubs must be spaced a minimum distance apart to allow clearance between circuits arranged on adjacent daughter boards as well as to achieve SCSI performance. It is desirable that board manufacturers and personal computer (PC) manufacturers maintain connector stub separation as small as possible, i.e., just above the minimum guidelines. Connector stub separation beyond the minimum would add material cost to the overall board. When a mere fraction of a dollar on each control board provides an advantage in the cost competitive PC marketplace, manufacturers must target stub separation at SCSI minimum. The trace conductors which interconnect connector stubs must therefore achieve the SCSI impedance mandate but only within the limited area between stubs.